# signal

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< c‎ | program

Defined in header ``<signal.h>`` |  |
void (*signal( int sig, void (*handler) (int))) (int); |  |

Sets the error handler for signal `sig`. The signal handler can be set so that default handling will occur, signal is ignored, or a user-defined function is called.

When signal handler is set to a function and a signal occurs, it is implementation defined whether signal(sig, SIG_DFL) will be executed immediately before the start of signal handler. Also, the implementation can prevent some implementation-defined set of signals from occurring while the signal handler runs.

### Parameters

sig | - | the signal to set the signal handler to. It can be an implementation-defined value or one of the following values:
SIGABRTSIGFPESIGILLSIGINTSIGSEGVSIGTERM | defines signal types (macro constant)
handler | - | the signal handler. This must be one of the following: SIG_DFL macro. The signal handler is set to default signal handler. SIG_IGN macro. The signal is ignored. pointer to a function. The signature of the function must be equivalent to the following:
void fun(int sig); |  |

### Return value

Previous signal handler on success or SIG_ERR on failure (setting a signal handler can be disabled on some implementations).

### Signal handler

The following limitations are imposed on the user-defined function that is installed as a signal handler.

If the user defined function returns when handling SIGFPE, SIGILL or SIGSEGV, the behavior is undefined.

If the signal handler is called as a result of abort or raise, the behavior is undefined if the signal handler calls raise.

If the signal handler is called NOT as a result of abort or raise (in other words, the signal handler is asynchronous), the behavior is undefined if

- the signal handler calls any function within the standard library, except

  - abort
  - _Exit
  - quick_exit
  - `signal` with the first argument being the number of the signal currently handled (async handler can re-register itself, but not other signals).
  - atomic functions from `<stdatomic.h>` if the atomic arguments are lock-free
  - atomic_is_lock_free (with any kind of atomic arguments)

- the signal handler refers to any object with static or thread-local(since C11) storage duration that is not a lock-free atomic(since C11) other than by assigning to a static volatile sig_atomic_t.

On entry to the signal handler, the state of the floating-point environment and the values of all objects is unspecified, except for

- objects of type volatile sig_atomic_t
- objects of lock-free atomic types (since C11)
- side effects made visible through atomic_signal_fence (since C11)

On return from a signal handler, the value of any object modified by the signal handler that is not volatile sig_atomic_t or lock-free atomic(since C11) is undefined.

The behavior is undefined if signal is used in a multithreaded program. It is not required to be thread-safe.

### Notes

POSIX requires that `signal` is thread-safe, and specifies a list of async-signal-safe library functions that may be called from any signal handler.

Besides `abort` and `raise`, POSIX specifies that `kill`, `pthread_kill`, and `sigqueue` generate synchronous signals.

POSIX recommends `sigaction` instead of `signal`, due to its underspecified behavior and significant implementation variations, regarding signal delivery while a signal handler is executed.

### Example

Run this code

    #include <signal.h>
    #include <stdio.h>

    volatile sig_atomic_t gSignalStatus;

    void signal_handler(int signal)
    {
      gSignalStatus = signal;
    }

    int main(void)
    {
      signal(SIGINT, signal_handler);

      printf("SignalValue: %d\n", gSignalStatus);
      printf("Sending signal: %d\n", SIGINT);
      raise(SIGINT);
      printf("SignalValue: %d\n", gSignalStatus);
    }

Output:

    SignalValue: 0
    Sending signal: 2
    SignalValue: 2

### References

- C17 standard (ISO/IEC 9899:2018):

  - 7.14.1.1 The signal function (p: 193-194)

- C11 standard (ISO/IEC 9899:2011):

  - 7.14.1.1 The signal function (p: 266-267)

- C99 standard (ISO/IEC 9899:1999):

  - 7.14.1.1 The signal function (p: 247-248)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.7.1.1 The signal function

### See also

raise | runs the signal handler for particular signal (function)
C++ documentation for signal

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